The ability of mycorrhizal fungi to ward off a wide variety of plant pathogens (such as nematodes) and diseases (Phytophthora, etc.) has been well documented. Any search will turn up dozens of studies on this topic.
However, it is not widely recognized that the beneficial fungi must have a chance to colonize the roots and surrounding soil before this important protective role can be realized. This argues for inoculating crop plants as early as possible - either at transplanting time or preferably while still in the nursery.
I am not aware of any research that indicates that mycorrhizal fungi can perform any recovery-type functions after a plant has been infected with disease or infested with harmful soil organisms. Its role in nature seems to be geared almost entirely toward the prevention of, rather than curing, plant problems.
There are at least four ways in which mycorrhizal fungi protect their host plants: 1) They create physical barriers around roots with sticky hyphae; 2) They produce antibiotic exudates that specifically target plant enemies; 3) They create positive hormonal changes in their host plant's immune system; 4) They provide greatly increased mineral uptake that leads to stronger plants better able to withstand diseases.
It is impossible to artificially replicate all the above good effects of mycorrhizal fungi with chemicals and synthetic fertilizers, which explains why crop plants tend to suffer from so many diseases and pathogen attacks as compared to non-cultivated plants. Without the normal and essential presence of their natural fungi partners on their root systems, crop plants are at a terrible disadvantage.
Moral: Give your plants the symbiotic protection they require for good health, and give it to them BEFORE they develop problems.
For good growing,
President, BioOrganics, Inc.